Dr. PH Martins Hydrus Liquid Watercolors are brilliant, lightfast, non-toxic, archival and AP approved liquid watercolors in 1 oz. dropper bottles. The dropper bottle allows you to repeatedly mix exact color shades by mixing the same number of drops each time. In addition to traditional watercolor painting techniques Dr. PH Martins Hydrus Liquid Watercolors are great for calligraphy dip pens, and airbrush. These are pigment based paints made with lightfast pigments such as quinacridone and Phthlocyanine. Like other watercolors, Hydrus Liquid Watercolors are non-waterproof. A popular technique is to paint them over the top of Dr. PH Martin Bombay India Inks, which are waterproof when dry.